The present invention relates to a brake slip control apparatus fed with auxiliary energy for use in a vehicular hydraulic brake system. The brake system comprising a master cylinder assembly actuatable by the brake pedal and incorporating a master cylinder piston and an assigned prechamber, with a supply bore and with the delivery of auxiliary energy into the working chamber of the master cylinder. At least one positioning tube which is axially slidable between stops and sealed relative to the bore in the master cylinder housing is arranged on the secondary side of the master cylinder piston. The positioning tube has a larger external diameter than the master cylinder piston and is centrically guided through the shank or the actuating rod of the master cylinder piston in a sealed relationship thereto. The master cylinder piston is caused to assume a defined position on actuation of the brake pedal and in the event of the secondary side of the master cylinder piston being applied by auxiliary energy during slip control.
In a brake slip control apparatus fed with auxiliary energy for use in a vehicular hydraulic brake system (brake system with anti-lock control) of the above-referenced type priorly known from German laid-open print No. 23 43 545, the master cylinder assembly is designed with a tandem master cylinder whose pedal-side (stepped) piston comprises a piston rod with a pedal-side one-piece, circumferential flange, the external diameter of the circumferential flange being larger than the external diameter of the assigned master cylinder piston. Intermediate of a brake valve assembly for supply of auxiliary energy, which brake valve assembly is actuatable by the brake pedal and inserted upstream of the tandem master cylinder, and the circumferential flange is a travel-responsive spring which, on brake application, supplies the vehicle operator with a feedback or feeling for braking. During a normal braking action, the auxiliary energy is supplied through the brake valve of the brake valve assembly to the prechambers of the tandem master cylinder and from there to the secondary side of the master cylinder pistons. When acted upon by auxiliary energy, the circumferential flange having a larger diameter than the master cylinder piston and providing sealing to the outside provides that in normal operation both master cylinder pistons are kept in the their initial position, the circumferential flange bearing against an axial stop close to the pedal and formed fast with the housing. The hydraulic pressure fluid delivered to the secondary side propagates through axial fill bores at the master cylinder piston to the working chambers of the cylinder and from there, subjected to dynamic control, to allocated wheel brake cylinders. On failure of the auxiliary-energy supply system, the circumferential flange of the master cylinder piston close to the pedal will no longer be held at its axial stop formed fast with the housing, so that both master cylinder pistons can move jointly in the actuating direction, the master cylinder performing its normal function then and actuating the wheel brake cylinders statically. What is a disadvantage in this known arrangement is the complicated design of the pedal-side master cylinder piston having an enlarged pedal-side circumferential flange of larger diameter. Since the piston is locked in its initial position during normal operation, there must be provided means in the form of a travel-responsive spring which gives the vehicle operator a good sense of braking. Inherently, the brake valve assembly itself does not suit this purpose. As the master cylinder piston is stationary during normal operation (and does not perform its normal function), whether said cylinder has any defective points (such as possible leakages in the area of the sleeve seal) cannot be detected during normal operation of the brake.
Furthermore, a vehicular brake system of the type at issue is known in U.S. Pat. No. 4,416,491, wherein on the secondary side of the master cylinder piston there is provision of a positioning piston which is axially slidable against an axial stop formed fast with the housing and which is sealed relative to the master cylinder housing, the positioning piston having a larger external diameter than the master cylinder piston and through which the master cylinder piston rod is centrically guided in a sealed relationship therewith, wherein on depression of the brake pedal and on application of the secondary side of the master cylinder piston with auxiliary energy during slip control, the pedal-side enlarged circumferential end of the master cylinder piston rod or a circumferential projection of a throttle valve assembly, which is possibly inserted upstream, is movable into engagement with an assigned pedal-side circumferential edge of the positioning piston such as to cause the master cylinder piston to assume a defined position during braking with slip control.
However, this known vehicular brake system has the disadvantage that during control action there is the inherent risk that, when braking is effected at low frictional values, the brake pedal will be displaced in the direction of the master cylinder so far until the booster piston abuts on the positioning tube.
It is an object of the present invention to improve upon the known positioning device such as to ensure on control action even under extreme conditions that the brake pedal assumes a position which is precisely defined beforehand.